1. Field of the Invention
The invention relates generally to desaturase enzymes that modulate the number and location of double bonds in long chain poly-unsaturated fatty acids (LC-PUFA's). In particular, the invention relates to improvement of fatty acid profiles using desaturase enzymes and nucleic acids encoding such desaturase enzymes.
2. Description of the Related Art
The primary products of fatty acid biosynthesis in most organisms are 16- and 18-carbon compounds. The relative ratio of chain lengths and degree of unsaturation of these fatty acids vary widely among species. Mammals, for example, produce primarily saturated and monounsaturated fatty acids, while most higher plants produce fatty acids with one, two, or three double bonds, the latter two comprising polyunsaturated fatty acids (PUFA's).
Two main families of PUFAs are the omega-3 fatty acids (also represented as “n-3” fatty acids), exemplified by eicosapentaenoic acid (EPA, 20:4, n-3), and the omega-6 fatty acids (also represented as “n-6” fatty acids), exemplified by arachidonic acid (ARA, 20:4, n-6). PUFAs are important components of the plasma membrane of the cell and adipose tissue, where they may be found in such forms as phospholipids and as triglycerides, respectively. PUFAs are necessary for proper development in mammals, particularly in the developing infant brain, and for tissue formation and repair.
Several disorders respond to treatment with fatty acids. Supplementation with PUFAs has been shown to reduce the rate of restenosis after angioplasty. The health benefits of certain dietary omega-3 fatty acids for cardiovascular disease and rheumatoid arthritis also have been well documented (Simopoulos, 1997; James et al., 2000). Further, PUFAs have been suggested for use in treatments for asthma and psoriasis. Evidence indicates that PUFAs may be involved in calcium metabolism, suggesting that PUFAs may be useful in the treatment or prevention of osteoporosis and of kidney or urinary tract stones. The majority of evidence for health benefits applies to the long chain omega-3 fats, EPA and docosahexanenoic acid (DHA, 22:6) which are in fish and fish oil. With this base of evidence, health authorities and nutritionists in Canada (Scientific Review Committee, 1990, Nutrition Recommendations, Minister of National Health and Welfare, Canada, Ottowa), Europe (de Deckerer et al., 1998), the United Kingdom (The British Nutrition Foundation, 1992, Unsaturated fatty-acids—nutritional and physiological significance: The report of the British Nutrition Foundation's Task Force, Chapman and Hall, London), and the United States (Simopoulos et al., 1999) have recommended increased dietary consumption of these PUFAs.
PUFAs also can be used to treat diabetes (U.S. Pat. No. 4,826,877; Horrobin et al., 1993). Altered fatty acid metabolism and composition have been demonstrated in diabetic animals. These alterations have been suggested to be involved in some of the long-term complications resulting from diabetes, including retinopathy, neuropathy, nephropathy and reproductive system damage. Primrose oil, which contains γ-linolenic acid (GLA, 18:3, Δ6, 9, 12), has been shown to prevent and reverse diabetic nerve damage.
PUFAs, such as linoleic acid (LA, 18:2, Δ9, 12) and α-linolenic acid (ALA, 18:3, Δ9, 12, 15), are regarded as essential fatty acids in the diet because mammals lack the ability to synthesize these acids. However, when ingested, mammals have the ability to metabolize LA and ALA to form the n-6 and n-3 families of long-chain polyunsaturated fatty acids (LC-PUFA). These LC-PUFA's are important cellular components conferring fluidity to membranes and functioning as precursors of biologically active eicosanoids such as prostaglandins, prostacyclins, and leukotrienes, which regulate normal physiological functions. Arachidonic acid is the principal precursor for the synthesis of eicosanoids, which include leukotrienes, prostaglandins, and thromboxanes, and which also play a role in the inflammation process. Administration of an omega-3 fatty acid, such as SDA, has been shown to inhibit biosynthesis of leukotrienes (U.S. Pat. No. 5,158,975). The consumption of SDA has been shown to lead to a decrease in blood levels of proinflammatory cytokines TNF-α and IL-1β (PCT US 0306870).
In mammals, the formation of LC-PUFA is rate-limited by the step of Δ6 desaturation, which converts LA to γ-linolenic acid (GLA, 18:3, Δ6, 9, 12) and ALA to SDA (18:4, Δ6, 9, 12, 15). Many physiological and pathological conditions have been shown to depress this metabolic step even further, and consequently, the production of LC-PUFA. To overcome the rate-limiting step and increase tissue levels of EPA, one could consume large amounts of ALA. However, consumption of just moderate amounts of SDA provides an efficient source of EPA, as SDA is about four times more efficient than ALA at elevating tissue EPA levels in humans (copending U.S. application Ser. No. 10/384,369). In the same studies, SDA administration was also able to increase the tissue levels of docosapentaenoic acid (DPA), which is an elongation product of EPA. Alternatively, bypassing the Δ6-desaturation via dietary supplementation with EPA or DHA can effectively alleviate many pathological diseases associated with low levels of PUFA. However, as set forth in more detail below, currently available sources of PUFA are not desirable for a multitude of reasons. The need for a reliable and economical source of PUFA's has spurred interest in alternative sources of PUFA's.
Major long chain PUFAs of importance include DHA and EPA, which are primarily found in different types of fish oil, and ARA, found in filamentous fungi such as Mortierella. For DHA, a number of sources exist for commercial production including a variety of marine organisms, oils obtained from cold water marine fish, and egg yolk fractions. Commercial sources of SDA include the plant genera Trichodesma, Borago (borage) and Echium. However, there are several disadvantages associated with commercial production of PUFAs from natural sources. Natural sources of PUFAs, such as animals and plants, tend to have highly heterogeneous oil compositions. The oils obtained from these sources therefore can require extensive purification to separate out one or more desired PUFAs or to produce an oil which is enriched in one or more PUFAs.
Natural sources of PUFAs also are subject to uncontrollable fluctuations in availability. Fish stocks may undergo natural variation or may be depleted by overfishing. In addition, even with overwhelming evidence of their therapeutic benefits, dietary recommendations regarding omega-3 fatty acids are not heeded. Fish oils have unpleasant tastes and odors, which may be impossible to economically separate from the desired product, and can render such products unacceptable as food supplements. Animal oils, and particularly fish oils, can accumulate environmental pollutants. Foods may be enriched with fish oils, but again, such enrichment is problematic because of cost and declining fish stocks worldwide. This problem is also an impediment to consumption and intake of whole fish. Nonetheless, if the health messages to increase fish intake were embraced by communities, there would likely be a problem in meeting demand for fish. Furthermore, there are problems with sustainability of this industry, which relies heavily on wild fish stocks for aquaculture feed (Naylor et al., 2000).
Other natural limitations favor a novel approach for the production of omega-3 fatty acids. Weather and disease can cause fluctuation in yields from both fish and plant sources. Cropland available for production of alternate oil-producing crops is subject to competition from the steady expansion of human populations and the associated increased need for food production on the remaining arable land. Crops that do produce PUFAs, such as borage, have not been adapted to commercial growth and may not perform well in monoculture. Growth of such crops is thus not economically competitive where more profitable and better-established crops can be grown. Large scale fermentation of organisms such as Mortierella is also expensive. Natural animal tissues contain low amounts of ARA and are difficult to process. Microorganisms such as Porphyridium and Mortierella are difficult to cultivate on a commercial scale.
A number of enzymes are involved in the biosynthesis of PUFAs. LA (18:2, Δ9, 12) is produced from oleic acid (OA, 18:1, Δ9) by a Δ12-desaturase while ALA (18:3, Δ9, 12, 15) is produced from LA by a Δ15-desaturase. SDA (18:4, Δ6, 9, 12, 15) and GLA (18:3, Δ6, 9, 12) are produced from LA and ALA by a Δ6-desaturase. However, as stated above, mammals cannot desaturate beyond the Δ9 position and therefore cannot convert oleic acid into LA. Likewise, ALA cannot be synthesized by mammals. Other eukaryotes, including fungi and plants, have enzymes which desaturate at the carbon 12 and carbon 15 position. The major polyunsaturated fatty acids of animals therefore are derived from diet via the subsequent desaturation and elongation of dietary LA and ALA.
Various genes encoding desaturases have been described. For example, U.S. Pat. No. 5,952,544 describes nucleic acid fragments isolated and cloned from Brassica napus that encode fatty acid desaturase enzymes. Expression of the nucleic acid fragments of the '544 patent resulted in accumulation of ALA. However, in transgenic plants expressing the B. napus Δ15-desaturase, substantial LA remains unconverted by the desaturase. More active enzymes that convert greater amounts of LA to ALA would be advantageous. Increased ALA levels allow a Δ6-desaturase, when co-expressed with a nucleic acid encoding for the Δ15-desaturase, to act upon the ALA, thereby producing greater levels of SDA. Because of the multitude of beneficial uses for SDA, there is a need to create a substantial increase in the yield of SDA.
Nucleic acids from a number of sources have been sought for use in increasing SDA yield. However, innovations that would allow for improved commercial production in land-based crops are still needed (see, e.g., Reed et al., 2000). Furthermore, the use of desaturase polynucleotides derived from organisms such as Caenorhabditis elegans (Meesapyodsuk et al., 2000) is not ideal for the commercial production of enriched plant seed oils. Genes encoding Δ6-desaturases have been isolated from two species of Primula, P. farinosa and P. vialii, and these found to be active in yeast, but the function in plants was not shown (Sayanova et al., 2003).
Therefore, it would be advantageous to obtain genetic material involved in PUFA biosynthesis and to express the isolated material in a plant system, in particular, a land-based terrestrial crop plant system, which can be manipulated to provide production of commercial quantities of one or more PUFA's. There is also a need to increase omega-3 fat intake in humans and animals. Thus there is a need to provide a wide range of omega-3 enriched foods and food supplements so that subjects can choose feed, feed ingredients, food and food ingredients which suit their usual dietary habits. Particularly advantageous would be seed oils with increased SDA.
Currently there is only one omega-3 fatty acid, ALA, available in vegetable oils. However, there is poor conversion of ingested ALA to the longer-chain omega-3 fatty acids such as EPA and DHA. It has been demonstrated in copending U.S. application Ser. No. 10/384,369 for “Treatment And Prevention Of Inflammatory Disorders,” that elevating ALA intake from the community average of 1/g day to 14 g/day by use of flaxseed oil only modestly increased plasma phospholipid EPA levels. A 14-fold increase in ALA intake resulted in a 2-fold increase in plasma phospholipid EPA (Manzioris et al., 1994). Thus, to that end, there is a need for efficient and commercially viable production of PUFAs using fatty acid desaturases, genes encoding them, and recombinant methods of producing them. A need also exists for oils containing higher relative proportions of specific PUFAs, and food compositions and supplements containing them. A need also exists for reliable economical methods of producing specific PUFA's.
Despite inefficiencies and low yields as described above, the production of omega-3 fatty acids via the terrestrial food chain is an enterprise beneficial to public health and, in particular, the production of SDA. SDA is important because, as described above, there is low conversion of ALA to EPA. This is because the initial enzyme in the conversion, Δ6-desaturase, has low activity in humans and is rate-limiting. Evidence that Δ6-desaturase is rate-limiting is provided by studies which demonstrate that the conversion of its substrate, ALA, is less efficient than the conversion of its product, SDA to EPA in mice and rats (Yamazaki et al., 1992; Huang, 1991).
Based on such studies, it is seen that in commercial oilseed crops, such as canola, soybean, corn, sunflower, safflower, or flax, the conversion of some fraction of the mono and polyunsaturated fatty acids that typify their seed oil to SDA requires the seed-specific expression of multiple desaturase enzymes, that includes Δ6-, Δ12- and/or Δ15-desaturases. Oils derived from plants expressing elevated levels of Δ6, Δ12, and Δ15-desaturases are rich in SDA and other omega-3 fatty acids. Such oils can be utilized to produce foods and food supplements enriched in omega-3 fatty acids and consumption of such foods effectively increases tissue levels of EPA and DHA. Foods and foodstuffs, such as milk, margarine and sausages, all made or prepared with omega-3 enriched oils, will result in therapeutic benefits. It has been shown that subjects can have an omega-3 intake comparable to EPA and DHA of at least 1.8 g/day without altering their dietary habits by utilizing foods containing oils enriched with omega-3 fatty acids. Thus, there exists a strong need for novel nucleic acids of Δ6-desaturases for use in transgenic crop plants with oils enriched in PUFAs, as well as the improved oils produced thereby.